Field of the Disclosure
Embodiments of the present invention relate to a liquid crystal display (LCD) device. More particularly, the embodiments relate to an LCD device that helps to overcome problems related to residual images.
Discussion of the Related Art
Because of advantages such as good portability, low power consumption, and low operating voltage, the LCD device is widely used in various fields such as notebook computers, monitors, spacecraft, aircraft, etc.
The LCD device includes a lower substrate, an upper substrate, and a liquid crystal layer formed between the lower and upper substrates. According to whether or not an electric field is applied, light transmittance is controlled by alignment of the liquid crystal layer, to thereby display images.
According to a method for controlling an alignment of the liquid crystal layer in the LCD device, the LCD device may be developed in various modes, for example, Twisted Nematic (TN) mode, Vertical Alignment (VA) mode, In-Plane Switching (IPS) mode, Fringe Filed Switching (FFS) mode, and etc.
Among the above modes, in case of the IPS mode and the FFS mode, both pixel and common electrodes are arranged on a lower substrate, whereby the alignment of liquid crystal layer may be controlled by an electric field between pixel and common electrodes.
In case of the IPS mode, pixel and common electrodes are alternately arranged in parallel, whereby a horizontal electric field is generated between the pixel and common electrodes, to thereby control the alignment of liquid crystal layer. In case of the FFS mode, pixel and common electrodes are provided at a predetermined interval by the use of insulating layer interposed therebetween, wherein one electrode is formed in a plate shape, and the other electrode is formed in a finger shape, whereby the alignment of liquid crystal layer is controlled by a fringe field occurring between the pixel and common electrodes.
Hereinafter, a related art IPS mode LCD device will be described.
FIG. 1 is a cross sectional view illustrating a related art IPS mode LCD device.
As shown in FIG. 1, the related art IPS mode LCD device may include an upper substrate 10, a lower substrate 20, a liquid crystal layer 30, and a sealant 40.
On the upper substrate 10, there are a light shielding layer 12, a color filter layer 14, an overcoat layer 16, and an upper alignment film 18 which are formed in sequence.
The light shielding layer 12 prevents light from leaking in other regions except pixel regions, wherein the light shielding layer 12 is formed in a matrix configuration. The color filter layer 14 is formed on the light shielding layer 12. The color filter layer 14 includes red (R), green (G), and blue (B) color filters. The overcoat layer 16 is formed on the color filter layer 14, wherein the overcoat layer 16 is provided to planarize the substrate. The upper alignment film 18 is provided for an initial alignment direction of the liquid crystal layer 30, and the upper alignment film 18 is aligned in a predetermined direction.
On the lower substrate 20, there are an array layer 22, a pixel electrode 24, a common electrode 26, and a lower alignment film 28.
Although not shown, the array layer 22 may include gate and data lines crossing each other to define the pixel region, and a thin film transistor formed adjacent to a crossing region of the gate and data lines. The pixel electrode 24 is formed on the array layer 22, and the pixel electrode 24 is electrically connected with the thin film transistor in the array layer 22. The common electrode 26 is formed on the array layer 22. Also, the common electrode 26 together with the pixel electrode 24 forms an electric field to drive the liquid crystal layer 30. The lower alignment film 28 is provided for an initial alignment direction of the liquid crystal layer 30, and the lower alignment film 28 is aligned in a predetermined direction.
The liquid crystal layer 30 is formed between the upper substrate 10 and the lower substrate 20. The liquid crystal layer 30 is provided in such a manner that its alignment state is controlled by a direction of electric field formed by the pixel electrode 24 and the common electrode 26.
The sealant 40 is formed between the upper substrate 10 and the lower substrate 20, wherein the sealant 40 adheres the upper substrate 10 and the lower substrate 20 to each other.
However, the related art IPS mode LCD device is disadvantageous in that a picture quality is deteriorated due to the change in properties of liquid crystal molecules included in the liquid crystal layer 30. That is, the liquid crystal molecules are formed of a mixture of various chemical substances. Some of the chemical substances may undesirably react to impurities included in the liquid crystal layer 30, whereby driving properties of the liquid crystal molecules may be changed, and thus residual images may remain when displaying images.
This will be described in detail as follows.
First, impurities may be produced for a process of preparing the liquid crystal molecules, and thus the impurities together with the liquid crystal molecules may be included in the liquid crystal layer 30. In this case, the liquid crystal molecules may react to the impurities by heat or ultraviolet (UV) rays, which may cause the change in properties of liquid crystal molecules.
Examples of the impurities produced for the process of preparing the liquid crystal molecules are shown as follows. Generally, liquid crystal compounds are made by reaction of intermediate products. If the liquid crystal compounds corresponding to end products are not produced by 100% reaction of the intermediate products, the liquid crystal layer 30 is formed by the mixture of the intermediate products and the liquid crystal compounds corresponding to the end products. In this case, the intermediate products function as the impurities, which causes the change on properties of the liquid crystal molecules.
Also, components included in elements for the LCD device, for example, the light shielding layer 12, color filter layer 14, overcoat layer 16, upper alignment film 18, lower alignment film 28, and sealant 40 may come into contact with the liquid crystal layer 30 due to various reasons, that is, the above components may serve as the impurities. Accordingly, the properties of liquid crystal molecules may be changed by the reaction between the impurities and the liquid crystal molecules.
FIG. 2 is a table showing the substances corresponding to the components included in elements for the LCD device, which come into contact with the liquid crystal layer 30, and thus serve as the impurities in the liquid crystal layer 30. As shown in FIG. 2, various types of substances may come into contact with the liquid crystal layer 30, and cause the change in properties of the liquid crystal molecules.